Ignore failure of "clean" when building extensions

Some extensions do not have a clean task, even though they go through
the extconf.rb builder.  Ignore failure of the clean task for these
gems.
Revert "Do not clean for cmake"

This commit is no longer needed after @184e257 as a failing clean task
is ignored.

This reverts commit 010ae97b5cb0d170cce74e4abe77699bb209492c.